1. Field of the Invention
The present invention relates to a photo-semiconductor electrode, a photoelectric converting device and a photoelectric conversion method. More particularly, the present invention relates to a photo-semiconductor electrode capable of converting light energy into electrical energy, a photoelectric converting device and a photoelectric conversion method.
2. Description of the Related Art
The global warming due to the burning of fossil fuels and the increase in the demand for energy due to the increase in population are now posing serious problems that relate to the existence of mankind. Needless to say, from time immemorial, solar light has supported global environments and is an energy source for all living things including mankind.
Recently, attempts have been made to use solar light as a limitless source of energy which is clean and not associated with the generation of hazardous substances. Well known devices which directly convert light energy into electrical energy, are solar cells. They are made of p-n junctions formed on inorganic semiconductors such as silicon or gallium arsenide. They have been put to practical use as the power source of, for example, remote control or portable electronic apparatus.
However, in order for these solar cells to be used as an energy source, a difficult problem needs to be solved. The problem is that a large amount of energy and cost is necessary for the production of these solar cells, although these solar cells have a high conversion efficiency.
Another method for converting light radiant energy into electrical energy is seen in the wet-process solar cell which utilizes a photo-electrochemical reaction occurring in an interface between a semiconductor and an electrolyte solution. An oxide semiconductor, such as titanium oxide or tin oxide, which is used in the wet-process, is expected to be a useful material for the conversion, because the oxide semiconductor can be produced with a far lower amount of energy and cost in comparison with the aforementioned semiconductors such as silicon or gallium arsenide. However, the problem of the photo-semiconductors such as titanium oxide and zinc oxide which are stable is that a high conversion efficiency of energy cannot be expected because the band gap of these semiconductors is as broad as 3 eV and therefore these semiconductors are capable of utilizing only ultraviolet light. This accounts for only about 4 percent of sunlight.
Accordingly, it is reported that the photoelectric energy conversion efficiency can be increased by using as a photo-electrode a modified electrode prepared by natural adsorption onto the surface of the photo-electrode of a substance which has a small band gap and which is exemplified by an organic dye such as a xanthene dye or a cyanine dye and a transition metal complex such as tris(2,2'-bipyridyl)ruthenium(II) (see, for example, T. Osa, M. Fujihira, ibid., 264, 349, (1976), Brian O'Regan, Michael Gratzel, Nature, 353, 736 (1991), and Japanese Patent Application Laid-Open (JP-A) No.1-220,380).
However, the above-described electrode, which was prepared by natural adsorption of a dye onto the surface of the photo-electrode, presented the problem that chemical stability and electrochemical stability were insufficient and the durability was also insufficient because the adsorbed dye was liable to come off.
In order to solve the problem of the prior art, a method was proposed which comprised fixing the dye on the surface of a semiconductor via y-aminopropyltriethoxysilane (JP-A No. 55-124,964). This method, however, still has a problem that little of the pigment is fixed to the surface of the semiconductor, and the function as an electrode is insufficient.
The task of the present invention is to achieve the following object by solving the above-described problems of prior art.
That is, the object of the present invention is to provide a photo-semiconductor electrode which is capable of efficiently absorbing solar light and performing energy conversion and which is superior in photoelectric conversion efficiency, stability and durability and can be easily produced. Another object is to provide a photoelectric conversion method using the photo-semiconductor electrode and a photoelectric converting device suitable for the implementation of the photoelectric conversion method.